See <http://searchfox.org/mozilla-central/source/embedding/components/windowwatcher/nsWindowWatcher.cpp#1118> This is definitely the wrong thing to do for expanded principals, as demonstrated by bug 1297687. It is also the wrong thing to do with the system principal. Since this code can run from the scriptable nsIWindowWatcher.openWindow(), it can cause bugs with add-ons calling it from such principals mentioned above.

Comment on attachment 8788959 [details] [diff] [review] Avoid inheriting the origin attributes of the subject principal if it's expanded That doesn't address subjectPrincipal being system. Why is that ok? r=me with that explained.

Err, it's not OK. I meant to do that in a separate patch for easier bisectability but I forgot. Sorry! :/ Filed as bug 1301201.

Pushed by eakhgari@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/fdfb1d87051b Avoid inheriting the origin attributes of the subject principal if it's expanded; r=bzbarsky